Copper and Cast Iron

Q I live in a 1920s Wardman row house in Washington, D.C. The house has a cast-iron radiator heating system. I hired a local contractor to do some bathroom renovations. It was necessary to remove a large cast-iron radiator from the bathroom to make room for the remodel. The contractor hired a plumber who installed a copper, baseboard-style radiator. It looked great! But the problem is that it only ekes out a minimal amount of heat. All of the other cast-iron radiators in the house sizzle with heat, and this copper baseboard heater is tepid at best. I called the plumber to come out to address the problem. He re-bled the system and said that it was fixed and working correctly. However, we noticed no appreciable change whatsoever. What can be done to resolve our problem? It’s December, and we need heat in the bathroom!
Tempted to Stay in BedWashington, D.C.